Faculty Profiles - KOJIMA Hiroki

Papers - KOJIMA Hiroki

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Discharge Characteristics of Composite Insulation System with Floating Electrode and Solid Insulator in Vacuum Reviewed International journal

F. Kong, Y. Nakano, H. Kojima, N. Hayakawa, T. Kimura, M. Tsukima

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 2 ) page： 1219 - 1225 2016.4

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

For the development of vacuum interrupters for their higher voltage application in power transmission systems, it is necessary to clarify the discharge characteristics of composite insulation systems with a shield (floating electrode) and a solid insulator in vacuum. In this paper, we focus on the composite discharge patterns via the shield and the solid insulator. For cathode-shield-insulator-anode (c-s-i-a) and cathode-insulator-shield-anode (c-i-s-a) electrode configurations, the discharge can be classified into two independent processes: breakdown in vacuum gap and surface flashover on the solid insulator. Furthermore, we have found that the breakdown development time increases with the increase in the gap length and the surface flashover development time depends on the voltage peak after flashover inception, which are consistent with discharge development characteristics for individual gap breakdown and surface flashover, respectively. These results are significant to understanding and discriminating the composite discharge patterns and discharge path in vacuum interrupters.

DOI： 10.1109/TDEI.2015.005588

Pressure Dependence and Size Effect of LN₂ Breakdown Characteristics Under Transient Thermal Stress Reviewed International journal

N. Hayakawa, T. Matsuoka, K. Ishida, H. Kojima, S. Isojima, M. Kuwata

IEEE Transactions on Applied Superconductivity Vol. 26 ( 3 ) page： 7700604 2016.4

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Electrical insulation of liquid nitrogen (LN2) for a high-temperature superconducting (HTS) power apparatus can be categorized into two breakdown (BD) modes. One is the intrinsic or conventional BD of LN2. The other is the BD of LN2 with transient bubbles, such as a quench. In this paper, the BD characteristics of LN2 with transient bubbles were investigated for different LN2 pressures and electrode diameters. Experimental results revealed that the BD strength of LN2 with transient bubbles was lower than the intrinsic BD strength of LN2. The difference between the intrinsic BD strength and the BD strength with transient bubbles in LN2 became smaller with the increase in the LN2 pressure and electrode diameter. Furthermore, the BD characteristics of LN2 with continuous bubbles were investigated and compared with the BD characteristics of LN2 with transient bubbles. As a result, the insulation design of an HTS power apparatus in consideration of their quench condition will be rationalized than the design based on the continuous thermal stress.

DOI： 10.1109/TASC.2016.2532469

Effective Secondary Electron Emission Coefficient under Transient Surface Charging on Insulator in Surface Flashover Development in Vacuum Reviewed

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 136 ( 3 ) page： 128 - 134 2016.3

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

Based on the mechanism of surface flashover development in vacuum, we have investigated that the surface flashover can be suppressed by the increase in electric field perpendicular to the insulator surface. In this study, we clarified the suppression conditions of surface flashover in terms of the transient surface charge and the effective secondary electron emission coefficient along the insulator surface.

DOI： 10.1541/ieejfms.136.128

Reduction Method of Fluctuation in Partial Discharge Inception Voltage for Inverter-fed Rectangular Wire Motors Reviewed

T. Wakimoto, H. Kojima, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 136 ( 3 ) page： 121 - 127 2016.3

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

We investigated the decisive factors which affect partial discharge inception voltage (PDIV) fluctuation between the coil wires under a sinusoidal voltage by using a rectangular wire motor. As the result, we found that PDIV under low humidity fluctuates by the surface charge, and PDIV under high humidity varies transitionally with the surface conductivity activated by pre-discharge. From these analyses, we proposed the measurement method of PDIV with a low fluctuation and transition for the rectangular wire motors, which will be optimized by environmental humidity, pre-discharge condition and measuring frequency.

DOI： 10.1541/ieejfms.136.121

Fabrication and Simulation of Permittivity Graded Materials for Electric Field Grading of Gas Insulated Power Apparatus Reviewed International journal

N. Hayakawa, J. Ishiguro, H. Kojima, K. Kato, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 547 - 554 2016.2

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

An innovation for the higher performance and reliability of electric power apparatus can be brought about by a novel technique on electrical insulating materials. The application of functionally graded materials (FGM) with spatial distribution of dielectric permittivity in solid insulator is an effective technique to contribute to the above innovation, because FGM can control the electric field distribution with a simple shape. In this paper, we investigated the fabrication and simulation techniques of FGM with various spatial distributions of dielectric permittivity using centrifugal force. Firstly, we developed a simulation technique to estimate the dielectric permittivity distribution of FGM by considering the particle movement in a viscous fluid under centrifugal force. Secondly, based on the simulation technique, we fabricated FGM samples with various relative permittivity distribution and confirmed the validity of the simulation technique. Consequently, the fabrication results provided a good agreement with the simulation results.

DOI： 10.1109/TDEI.2015.005237

Classification of Impulse Breakdown Mechanisms under Non-uniform Electric Field in Air Reviewed International journal

H. Kojima, K. Hotta, T. Kitamura, N. Hayakawa, A. Otake, K. Kobayashi, T. Kato, T. Rokunohe, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 194 - 201 2016.2

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

To clarify the breakdown mechanism and the conditions leading to the breakdown under non-uniform electric field in air, we studied the details of breakdown characteristics and mechanism in air by using an ultra-high speed measurement system. We clarified that there were two types of breakdown mechanism in air. The one is the channel heating breakdown, and the other is the leader-Type channel development breakdown. These breakdown mechanisms are dominated by three conditions: Arrival of secondary streamer from high-voltage anode to opposite grounded cathode, channel heating, and leader-Type channel initiation. We studied these conditions quantitatively, and revealed the conditions leading to the breakdown, and the breakdown mechanisms in air were classified. Finally, we clarified the factors, e.g. critical instantaneous power for the secondary streamer arrival, dominating these conditions based on the breakdown mechanism under non-uniform electric field in air.

DOI： 10.1109/TDEI.2015.005352

Analysis of Conditioning and Damaging Process in Vacuum based on Breakdown Probability Distribution Reviewed International journal

H. Kojima, Y. Otake, T. Takahashi, N. Hayakawa, K. Hasegawa, H. Saito, M. Sakaki

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 43 - 48 2016.2

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

For the enhancement of insulation strength in vacuum, spark conditioning is an important and effective method. In order to enhance the insulation performance of vacuum circuit breakers, we have to clarify the conditioning effect and improve a conditioning procedure. In this paper, we discuss the conditioning process of Cu and stainless steel rod electrodes based on breakdown (BD) probability distribution. We applied a negative impulse voltage to a rod-plane electrode configuration. We adopted two procedures for the voltage application, i.e. up-down and 3 times methods. For the Cu rod with the up-down method, BD voltage increased by the conditioning effect, however, the minimum BD voltage did not increase or even decreased due to the damaging effect by BD. A moderate conditioning procedure, e.g. 3 times method, could increase the minimum BD voltage as well as the stabilization of insulation strength. The Weibull analysis of the development of a BD probability distribution has enabled us to discriminate the conditioning and damaging effects. Based on these results, we can discuss a suitable conditioning procedure.

DOI： 10.1109/TDEI.2015.005248

Control and Suppression of Impulse Surface Flashover in Vacuum Based on Discharge Development Mechanisms Reviewed International journal

Y. Nakano, H. Kojima, S. Iitsuka, K. Tsuchiya, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 23 ( 1 ) page： 35 - 42 2016.2

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Surface discharge on solid insulator in vacuum is one of the significant issues for the development of vacuum circuit breakers (VCBs) and vacuum interrupters (VIs). For the development of higher voltage VCBs and VIs, control and suppression techniques of the surface flashover are required. The aim of present study is to suppress the surface flashover, based on physical mechanisms of flashover development. Impulse surface flashover in vacuum, starting with explosive electron emission (EEE) from cathode, develops with the formation of transient charge on the insulator as well as the development of secondary electron emission avalanche (SEEA) along the surface toward anode. To reduce the EEE current from the cathode and the SEEA development, we changed the electrode geometry such as thickness of alumina ceramics and tip radius of cathode rod, respectively, and installed a plane electrode as the back electrode of the cathode rod. As the results, EEE current was reduced by the field relaxation effect of transient charging due to EEE, and SEEA was also suppressed by the perpendicular electric field of the plane electrode. When the flashover was suppressed by using the cathode back electrode, transient charge amount was smaller on alumina dielectrics at the end of the plane electrode area. From the above results, it would be an effective technique for the control and suppression of surface flashover in vacuum to enlarge the perpendicular electric field for the insulator.

DOI： 10.1109/TDEI.2015.005307

Transient Charging Characteristics in Surface Flashover Development Process due to Explosive Electron Emission in Vacuum Invited International journal

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

Electrical Engineering in Japan Vol. 193 ( 2 ) page： 10 - 17 2015.11

Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley Periodicals, LLC.

For the development of higher voltage vacuum circuit breakers and vacuum interrupters, surface discharge in a vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in a vacuum through ultrahigh-speed optical and electrical measurements. We found that surface flashover in a vacuum starts with an explosive electron emission (EEE) with higher current density, which consists of electron emission from the cathode, and results in surface flashover development. The transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. In particular, in the case of a shorter gap between the cathode rod and the alumina dielectrics, the electron emission may be suppressed by the relaxation effect of the electric field due to transient charge accumulation.

DOI： 10.1002/eej.22739

Size Effect on Breakdown Strength in Sub-cooled Liquid Nitrogen for Superconducting Power Apparatus Reviewed International journal

N. Hayakawa, S. Nishimachi, H. Kojima, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 22 ( 5 ) page： 2565 - 2571 2015.10

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Toward the practical and rational insulation design of high temperature superconducting power apparatus, the ac breakdown (BD) characteristics and mechanisms of liquid nitrogen (LN2) were systematically investigated and discussed for different pressures at P = 0.1-0.3 MPa and temperatures at T = 65-77 K under quasiuniform electric field with the field utilization factor ζ= 0.986-0.674. We obtained the BD strength (EB) in sub-cooled LN2 (SLN2) and analyzed them in terms of size effect, taking account of the stressed liquid volume (SLV) with the electric field distribution in SLN2. Together with the other researchers' data on BD characteristics of LN2, we extended the evaluation by applying the concept of α% SLV (SLV with the electric field strength higher than the decisive electric field factor, α of the maximum electric field strength). As the result, we derived that EB with the 50 % probability decreased linearly with the increase in α% SLV on a universal line, EB = 78.4 × (α% SLV)-1/8.15, which was irrespective of the pressure and temperature of LN2.

DOI： 10.1109/TDEI.2015.005089

Impulse Surface Flashover Development Associated with Transient Charging by Explosive Electron Emission in Vacuum Reviewed International journal

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 22 ( 4 ) page： 2390 - 2397 2015.8

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Surface discharge on solid insulator in vacuum, which is one of the most significant issues under high electric field, prevents the development of higher voltage vacuum circuit breakers and vacuum interrupters. To clarify the mechanism of the surface discharge in vacuum, charge activities on solid insulators in the development process of the surface discharge should be investigated. By using electrostatic probes behind the alumina dielectrics and measuring fast-framing images of the surface discharge, in the present paper, we clarified that the impulse surface flashover starts in developing with explosive electron emission (EEE) on cathode, which is specific phenomenon on the electron emission site exploding due to extreme field emission current density. Experimental results revealed the characteristics of transient charging by EEE in the surface flash over development as follows: (1) the probe current due to EEE increases with the EEE inception electric field on the cathode, and also depends on the work function of the cathode material, (2) the transient charge distribution on the alumina dielectrics is determined by the EEE spot on the cathode, (3) EEE occurs several times due to the erosion and the regeneration of electron emission sites. (4) the successive EEE makes the transient charge distribution smooth on the alumina dielectrics.

DOI： 10.1109/TDEI.2015.004948

Fault Current Limitation Coordination in Power Transmission System With Superconducting Fault Current Limiting Cables (SFCLC) Reviewed International journal

H. Kojima, T. Osawa, N. Hayakawa

IEEE Transactions on Applied Superconductivity Vol. 25 ( 3 ) page： 5401904 2015.6

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

We investigate a Superconducting Fault Current Limiting Cable (SFCLC), which is an HTS cable with fault current limiting function. From the viewpoint of the fault current limitation coordination in a power transmission system, it is necessary to investigate the interaction of current limiting functions between multiple SFCLCs introduced in the system. We constructed a numerical model to analyze electrical and thermal behaviors of SFCLC based on resistance generation characteristics of superconductors. In the model power system, two SFCLCs were connected in parallel and two fault points were assumed. We calculated the current limiting characteristics of each SFCLC in the model system, and evaluated the current limitation effect and temperature rise of SFCLCs on each fault point. Depending on the fault point, there are two types of substantial connection of SFCLCs, namely in series and in parallel during the fault. In the substantially parallel connection, each SFCLC operates as the case of single SFCLC introduction. In the substantially series connection, the current limiting effect is determined by SFCLC with the smaller critical current. These results imply that the efficiency and reliability of a power transmission system will be improved by the introduction of multiple SFCLCs, as well as the single SFCLC.

DOI： 10.1109/TASC.2015.2392251

Quench-Induced Dynamic Breakdown Characteristics of LN₂ and LN₂/PPLP Composite Electrical Insulation System for Superconducting Power Apparatus Reviewed International journal

N. Hayakawa, T. Matsuoka, S. Nishimachi, H. Kojima

IEEE Transactions on Applied Superconductivity Vol. 25 ( 3 ) page： 7700304 2015.6

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

In HTS power apparatus, quench due to the fault current generates dynamic electrical/thermal combined stress. This stress may induce the electric breakdown (BD) under the operating voltage. We investigated the quench-induced dynamic BD characteristics of LN2. In addition we investigated the quench-induced dynamic partial discharge characteristics of LN2/polypropylene laminated paper composite electrical insulation system for HTS cables. The criteria of the combined electrical/thermal stress for the dynamic BD were obtained for both test samples, and the reduction rate of the dynamic BD criteria against the static BD criteria without thermal stress was discussed.

DOI： 10.1109/TASC.2014.2361405

Power Flow Control and Maintenance Strategy of Transformers in Transmission and Distribution Systems Reviewed

H. Kojima, K. Fukaya, N. Hayakawa, M. Hanai, H. Okubo

IEEJ Transactions on Power and Energy Vol. 135 ( 5 ) page： 316 - 321 2015.5

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

Transmission and distribution (T&D) systems for electric power are assembled with a large amount of aged apparatus, the deterioration of which may cause a decrease in reliability. In order to maintain high efficiency and high quality in T&D systems, it is important to minimize both present performance costs and future performance costs. Typical present performance costs are T&D losses, and typical future performance costs are failure losses and apparatus maintenance for overall T&D system optimization. We have to consider concurrently two concepts that are an asset management system and a smart grid. This integrated concept, which we have proposed as Intelligent Grid Management System (IGMS), achieves optimum control that includes the present power flow and the future maintenance of the T&D system. In this paper, the effect of temperature rises in transformer windings is formulated and integrated into an objective function to determine transformers with power flow control. The derived power flow control optimizes the load sharing of each transformer by keeping a balance between the T&D loss and degradation in the transformers. The degradation of aged transformers is prevented by reducing their load factor, or the aged transformer is replaced with a new one that accepts a relatively high load factor.

DOI： 10.1541/ieejpes.135.316

Transient Charging Characteristics in Surface Flashover Development Process due to Explosive Electron Emission in Vacuum Reviewed

Y. Nakano, H. Kojima, K. Tsuchiya, N. Hayakawa

IEEJ Transactions on Fundamentals and Materials Vol. 134 ( 11 ) page： 578 - 584 2014.11

Language：Japanese Publishing type：Research paper (scientific journal) Publisher：The Institute of Electrical Engineers of Japan

For the development of higher voltage vacuum circuit breakers (VCB) and vacuum interrupters (VI), surface discharge phenomenon in vacuum is one of the most significant issues. We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed optical and electrical measurements. We found out that the surface flashover in vacuum starts with an explosive electron emission (EEE) with the higher current density, which is the electron emission from the cathode, and results in the surface flashover development. Transient charge on the alumina dielectrics formed by electrons emitted by EEE influences the distortion of electric field distribution around the cathode rod and then the subsequent flashover development. Especially, in the case of the shorter gap between the cathode rod and the alumina dielectrics, the electron emission could be suppressed by the relaxation effect of the electric field due to the transient charge accumulation.

DOI： 10.1541/ieejfms.134.578

Other Link： https://cir.nii.ac.jp/crid/1520853833628404608

Influence of Longitudinal Temperature Distribution on Current Limiting Function of Superconducting Fault Current Limiting Cable (SFCLC) Reviewed International journal

H. Kojima, T. Osawa, N. Hayakawa, M. Hanai, H. Okubo

Journal of Physics: Conference Series Vol. 507 ( PART 3 ) page： 032026 2014.5

Authorship：Lead author,　Corresponding author Language：English Publishing type：Research paper (scientific journal) Publisher：IOP Publishing

We have proposed a Superconducting Fault Current Limiting Cable (SFCLC), which is an HTS cable with fault current limiting function. SFCLC is expected to limit the fault current and also immediately recover the cable function after the fault clearance. In the SFCLC operation, a longitudinal temperature distribution will exist due to heat penetration, AC loss, dielectric loss and the performance of cryocooling system, which will influence its current limitation characteristics. In this paper, we investigate the influence of the longitudinal temperature distribution on current limiting function and temperature rise after the current limitation of SFCLC. We suggested the effective measures of parameter control, i.e. decreasing the critical current (Ic@77K), n value at flux flow region (n1-0), increasing the coefficient of longitudinal temperature gradient (α), inflow temperature (Tin) to achieve both the higher current limiting function and the lower temperature rise. © Published under licence by IOP Publishing Ltd.

DOI： 10.1088/1742-6596/507/3/032026

Technique for Discriminating the Type of PD in SF₆ Gas Using the UHF Method and the PD Current with a Metallic Particle Reviewed International journal

M. Hanai, H. Kojima, N. Hayakawa, R. Mizuno, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21 ( 1 ) page： 88 - 95 2014.2

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

The relationship between the PD current pulse waveform at the PD source and the radiated electromagnetic (EM) waveform is investigated using a needle electrode for a metallic particle in SF6 gas at 0.2MPa. A leader discharge with a larger PD charge magnitude and a longer propagation length than for a streamer discharge can be an indicator of breakdown in the SF6 gas. Simultaneous and synchronous measurement of the PD current, EM signals and light emission in the SF6 gas revealed that it is possible to discriminate between a streamer discharge and a leader discharge in both time and frequency components. Compared with a streamer discharge, a leader discharge is characterized by a longer rise/fall time of around 0.9 ns in the PD current pulse waveform, and by a lower frequency component of around 250 MHz in the EM frequency spectrum. A satisfactory correlation was obtained between the actual measured EM wave spectrum and the spectrum derived using a spectrum analyzer from the product of the actual PD current and the transfer function from the needle electrode to the UHF sensor that receives the EM wave. This new technique enables streamer and leader types of PD in SF6 gas to be discriminated between by measuring the EM wave with a UHF detection system. © 1994-2012 IEEE.

DOI： 10.1109/TDEI.2013.004021

Pre-discharge and Flashover Characteristics of Impulse Surface Discharge in Vacuum Reviewed International journal

Y. Nakano, H. Kojima, N. Hayakawa, K. Tsuchiya, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 21 ( 1 ) page： 403 - 410 2014.2

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Surface discharge on solid insulators is one of the significant issues in the development of vacuum circuit breakers (VCB) and vacuum interrupters (VI). We measured the charge activity on alumina dielectrics in vacuum through ultra-high speed and high sensitivity optical measurements. We identified the existence of charge activity with a small light emission on alumina dielectrics before the surface flashover development process, and defined this as a 'pre-discharge' and measured its characteristics. We classified the charge activities into three processes: (1) pre-discharge, (2) surface flashover initiation, and (3) surface flashover expansion, in the development of surface discharge in vacuum. Utilizing this classification, we found that the surface predischarge in vacuum is generated by the electron emission from the cathode, and the subsequent flashover initiates with a steep increase in current and develops with the distortion of the electric field due to transiently-formed charging on the alumina dielectrics in vacuum. © 2013 IEEE.

DOI： 10.1109/TDEI.2013.004136

Development of a Partial Discharge Inception Calculation Model of a Nonuniform Air Gap Considering the Effect of Humidity Invited International journal

N. Yanagita, T. Kato, T. Rokunohe, T. Iwata, H. Kojima, N. Hayakawa, H. Okubo

Electrical Engineering in Japan Vol. 186 ( 3 ) page： 1 - 9 2014.2

Language：English Publishing type：Research paper (scientific journal) Publisher：Wiley Periodicals, LLC.

Insulation design employing discharge simulation based on physics phenomena is used in designing electrical instruments to facilitate their miniaturization and obtain high insulation reliability. We developed a partial discharge inception calculation model for a nonuniform air gap based on the theories of electron avalanche and streamer phenomena. The calculated and experimental PDIVs (Partial Discharge Inception Voltages) were compared under conditions of constant humidity, and we confirmed that these agreed well with each other to within about 10%. Furthermore, we incorporated a relationship between negative ions and humidity into the model and confirmed that the calculated and experimental PDIVs agreed to within about 15%. Therefore, this model was judged to be an effective method for predicting changes in the PDIV when the humidity changes. © 2013 Wiley Periodicals, Inc.

DOI： 10.1002/eej.22351

Integration of Asset Management and Smart Grid with Intelligent Grid Management System Reviewed International journal

M. Hanai, H. Kojima, N. Hayakawa, K. Shinoda, H. Okubo

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 20 ( 6 ) page： 2195 - 2202 2013.12

Language：English Publishing type：Research paper (scientific journal) Publisher：IEEE

Electric power transmission and distribution (T and D) systems are composed of a great deal of aged apparatus, which may cause a decrease in reliability owing to their deterioration. In order to maintain high efficiency and high quality in T and D systems, the authors have proposed an "intelligent grid management system" (IGMS), which determines the optimum maintenance strategy and optimum power flow control based on condition monitoring and diagnostic results of the operating power apparatus. This means that the IGMS essentially includes both concepts of an asset management system and a smart grid. Further, the IGMS optimizes power flow routes and maintenance plans based on the failure risk, T and D loss, overload operation, life estimation of the power apparatus, customer outage, and other metrics. The impact of individual apparatus failure affects the entire T and D system's performance, causing blackouts and secondary failures. Reduction in reliability of whole system is highly dependent on ageing of the materials. The IGMS evaluates all of the events occurring in the T & D system as the cost. Additionally, the risks are evaluated in the cost according to the impact of the failure rate estimated by the condition monitoring diagnosis results of the power apparatus. Insulation system determines the transition in reliability and maintenance cost of the system. In this paper, the IGMS is applied to T and D system models including aged apparatus, such as transformers and circuit breakers, and suitable power flow routes and maintenance strategies are derived. Consequently, by the effective application of the IGMS, the system reliability can achieve an optimum state, and the total cost can be minimized. © 2013 IEEE.

DOI： 10.1109/TDEI.2013.6678870

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